The overall objectives of the proposed project are to investigate and clarify the pharmacological and physiological interrelationships among calcium and the secretion of calcemic hormones and gastrointestinal (GI) hormones. Principal attention sill be focussed on the physiological and pharmacological effects and mechanisms of action of calcium, thyrocalcitonin (TCT), parathyroid hormone (PTH), and gastrin (including related peptides, e.g., cholecystokinin) in an attempt to elucidate the role that these interrelationships might play in health and disease. Problems to be investigated include: 1. Mechanism by which calcium and GI hormones stimulate TCT secretion as measured by radioimmunoassay either in vivo in pig thyroid vein or rat cardiac blood or in vitro in media used to incubate pig or rat thyroid slices. 2. Possible influence of TCT and PTH, either directly or via blood calcium, on secretion of gastrin as measured by radioimmunoassay either in vivo in pigs and rats or in vitro in media used to incubate slices of pig or rat gastric antrum or duodenum. 3. Determination of whether calcium, GI hormones and other known secretagogues affect biosynthesis of TCT as well as its release (and, conversely, whether calcium, TCT or PTH affect gastrin synthesis) by in vitro incubation of pig and rat thyroid and gastric tissues with radiolabeled amino acids. Radioactive hormonal peptides will be separated from nonhormonal peptides using immunoadsorbents and/or standard chromatographic procedures and assessed by bioassay and rasioimmunoassay. 4. Development and application of a radioimmunoassay for rat TCT so that interrelationships among calcemic and GI hormones and calcium can be conveniently studied in a small animal species historically and extensively employed in the study of calcemic hormones, calcium homeostasis and skeletal metabolism.